opt out
You forgot to mention that Microsoft actually allowed you to opt out of this downgrade - it was just not publicised so most people didn't know about it.
Microsoft is emailing OneDrive users about its plan to shave 10GB from its 15GB free storage. From July 27, OneDrive users will get just 5GB of free storage in Microsoft's cloud. Microsoft is offering a year’s free subscription to Office 365 Personal with 1Tb storage for people over the 15GB threshold in response to the outcry …
I think that you have to go to https://preview.onedrive.com/bonus/ and sign up. However, if memory serves me right, you had to do this before the end of January. You may still be able to do this, it depends upon how forgiving Microsoft is to that deadline.
Forgiving may be a bit of a misnomer, given that they didn't publicise the downgrade much and positively hid the opt out (you really had to hunt for it). Good luck for getting the 15gb back. Microsoft have really been devious on this, for no real purpose it seems.